The present invention deals with a roll casting process whereby metal is continuously cast between cooled, counter rotating rolls, subsequently to emerge from the gap between the rolls as a solidified strip; The process includes providing a flow of coolant along the roll surface, in the direction of the roll gap and on both sides of the cast strip. The coolant is then drained off in the direction of the cast strip and along the cast strip so that the strip sticks to one of the rolls, which results in more intense cooling on the opposite side of the strip. This causes asymmetric heat tension in the strip with reference to its center line and thus creates a bending moment in the strip which causes a detachment of the strip from a sticking roll.
By means of so called roll casters the process referred to has found industrial application since the third decade of this century, its significance greatly increasing since 1955 (D. E. Herrmann, Handbook on Continuous Casting, 1980 Ed.)
The thickness of the cast strip resulting from systems built to date lies in the range of 3 to 5 mm, usually measuring 6 to 8 mm; and more recent production lines cast a strip measuring from 0.25 to 2 m in width. However, with appropriate dimensioning of the rolls, of their bearings, and of the drives, the casting process itself presents no limits as to the width of the strip being cast and it is quite feasible to cast strips with a width of 3 to 4 m.
The following description, referring as an example to the casting of aluminum, is also valid by adjustment of the corresponding data for analogous applications of the roll casting process to other materials, especially steel.
So far the process of roll casting has been mainly applied for the production of aluminum strips, allowing for an hourly production rate of 900 to 1200 kg per m of strip-width, depending on the thickness and the alloy of the cast strip. The strip thus cast emerges from the roll-gap with a speed, generally called casting speed, of 0.75 to 1.4 m/min. Having emerged from the rolls, the cast strip usually has a temperature of 300 to 400 degrees centigrade.
Any direction of casting is possible. We know of systems casting straight upward, horizontally or at an angle, be it upward or downward.
The rolls are combined with a cooling system allowing for the acquired heat to be carried off by means of a coolant. For this purpose, the internal cooling of the rolls has so far prevailed, the rolls being placed inside a shell and featuring grooves through which the coolant circulates. It is also possible, however, to use external systems whereby the surface of the rolls is directly contacted by the coolant and dried before reentering the casting zone (Sir Henry Bessemer, 1846).
Every applicant of the casting process strives to achieve the highest possible production rate, i.e. to run the system at the highest possible casting speed. It is required that no liquid metal passes through between the rolls, as this would interrupt the casting process or at least create strong disturbances until the breakthrough of liquid metal is stopped by varying of the casting parameters (decrease of casting speed and/or decrease of metal temperature in the feed system; cleaning of the roll surfaces etc.).
Since the required contact time between the rolls and the metal being cast is determined by the alloy and the thickness of the cast strip along with the thermal conditions (heat flow), it is reasonable to increase the length of contact between the rolls and the metal being cast by moving the nozzle back (increase of the distance h in FIG. 1) and at the same time increasing the casting speed without going below the necessary contact time.
Experience shows that solidification of the molten metal over the width of the cast strip can take place at somewhat differing speeds. This is caused by small variations in the heat flow due to temporal and/or local differences in the roll surface, e.g. as a result of the nozzle's rubbing on the rolls and/or variations of the temperature in the coolant or in the liquid metal or other cirumstances.
In order to avoid with all certainty a breakthrough of liquid metal, it is expedient to allow for a certain distance (distance a in FIG. 3) between the point of complete solidification of the cast metal and the point of emergence from between the rolls.
With today's casting speeds as mentioned above and with a thickness of the cast strip of approximately 6 mm (with reference to aluminum) a distance (h) of approximately 30 mm between nozzle aperture and emergence from between the rolls has proven to be appropriate (FIG. 3), the average distance (a) thereby amounting to approximately 12 mm. Due to the reasons mentioned above, this distance can vary within a range of approximately 8 to 16 mm across the width of the cast strip and in the course of time.
The process therefore includes a slight rolling effect after the complete solidification of the cast metal. Assuming for example a diameter of 600 mm for the rolls, the distance of a=12 mm will result in a reduction rate of 7.4%. With a local minimum of a=8 mm the reduction rate will amount to 3.4% and for the maximum of a=16 mm it amounts to 12.4%.
Experience shows that with this rolling effect on dry, non-lubricated rolls having very high surface-temperature the cast strip , while still soft has the tendency to stick to the rolls. The strip emerging from between the rolls has the basic tendency to move away from the rolls in the plane of symmetry. If the adhesion to one of the rolls is greater than to the other and if the difference exceeds a permissible value mainly dictated by the flexural strength of the strip at the point of emergence from between the rolls, the strip will stick to the one roll and must be loosened by force usually applied by means of scrapers or corresponding high strain in the strip. This strongly reduces the quality of the strip, to the effect that by today's high quality requirements it is rendered useless for most applications. To a certain extent the danger of sticking can be reduced by spraying the rolls with a readily evaporating liquid such as suspended graphite, molybdenum disulphide, boron nitride, magnesium oxide etc. which serve as stripping agents.
If for example the casting speed is 1.2 m/min and the distance between nozzle aperture and point of emergence from between the rolls h=30 mm (FIG. 3), the average contact time between cast metal and the rolls amounts to 1.5 s. This time is composed of the average time for solidification, 0.9 s (length of the solidification zone b=18 mm, FIG. 3) and the average rolling time, 0.6 s (length of the rolling zone a=12 mm, FIG. 3).
Considering a casting process in view of these durations it becomes obvious that an increase in casting speed with constant durations for the individual phases (solidification, rolling) requires an increase in the distances a, b and h (FIG. 3). Maintaining the same roll diameter, an increase in casting speed therefore results in an increase of the rolling effect and of the strip deformation. The resulting increased rolling pressure causes the strip to adhere more strongly to the rolls despite the application of above mentioned stripping agents, the permissible difference in adhesion between the strip and each of the rolls being exceeded at least from time to time, thus causing the strip to stick to one of the rolls and having to be loosened as described above by applying external force.